1. Field of the Invention
The present invention relates to an optical device to connect dielectric slab waveguides having different refractive indices.
2. Description of the Related Art
Optical integration technology has been initially studied in an optical communications and is recently been explored for use in various other fields. An optical waveguide, which is a type of device produced using optical integration technology, can be combined with conventional optical components such as lenses, prisms, mirrors, etc. Such an optical waveguide combined with the conventional optical components is being considered for use as part of an optical Dick-up device. Also, it is proposed that an optical waveguide can be applied to a polarizing isolator to detect optomagnetic signals.
In polarizing isolators and other optical integrated apparatuses, dielectric slab waveguides having different effective refractive indices may be connected with each other. Direct connection of two dielectric slab waveguides having different effective refractive indices causes reflected waves at a boundary therebetween. Existence of such reflected waves can result in loss of a light power at the boundary.
A connection of two dielectric slab waveguides having different effective refractive indices is now explained. FIG. 5A shows that a dielectric slab waveguide 105 is directly connected with a dielectric slab waveguide 107. Both dielectric slab waveguides 105 and 107 propagate single mode light. The dielectric slab waveguide 105 includes a portion of a dielectric substrate 103, a dielectric layer 102 formed on the portion of the dielectric substrate 103, and a dielectric layer 101 formed on the dielectric layer 102. The dielectric layers 102 and 101 have thicknesses of t.sub.102 and d.sub.101, respectively. The dielectric slab waveguide 107 includes another portion of the substrate 103 and a dielectric layer 104 formed on the other portion of the substrate 103. The dielectric layer 104 has a thickness of d.sub.104. As is represented in FIG. 5B, the dielectric slab waveguides 105 and 107 have effective refractive indices N.sub.105 and N.sub.107, respectively.
In the case where light I enters a boundary C between the dielectric slab waveguides 105 and 107 at an angle .THETA., a portion of the light I is transmitted through the boundary C into the dielectric slab waveguide 107 and another portion of light I is reflected at the boundary C. The reflected light portion designated by R is reflected away from the boundary C and returns back toward the dielectric slab waveguide 105. Thus, all of the light I is not propagated through the boundary C into the dielectric slab waveguide 107. This means that the transmission of light I loses part of the light power at the boundary C during traveling from the dielectric slab waveguide 105 to the dielectric slab waveguide 107. This is because that intensity distribution I.sub.in of the light in the dielectric slab waveguide 105 is different from intensity distribution I.sub.out of the light in the dielectric slab waveguide 107.
The loss of light power mentioned above is conventionally solved by forming a tapered layer 106 between the dielectric slab waveguide 105 and the dielectric slab waveguide 107, as is shown in FIG. 6A. The tapered layer 106 includes a dielectric layer 108, a dielectric layer 109, and a portion of the dielectric substrate 103 and has a length 1.sub.106. The layer 109 is formed in tapered shape which satisfies t.sub.102 /1.sub.106 &lt;1/10. As is shown in FIG. 6B, an effective refractive index N of the tapered layer 106 gradually changes in a z-axis direction. The tapered layer 106 enables gradual change of an intensity distribution of light incident on the tapered layer 106. The intensity distributions at z=0 and z=1.sub.106 correspond to intensity distributions in the dielectric slab waveguides 105 and 107, respectively. Therefore, the dielectric slab waveguides 105 and 107 are connected with each other without loss of the light power.
A tapered layer having a desired shape must be formed with accuracy so as to connect two dielectric slab waveguides by the aforementioned method. However, it is difficult to form a tapered layer having such a gradual slope. FIGS. 7A through 7C show a method of forming a layer with a gradual taper by a dry etching method utilized in a semiconductor manufacturing process. A layer 81 is formed on a substrate 3, and a resist pattern 82 is formed on the layer 81. An ion beam 83 is irradiated into the layer 81 at an irradiation angle .alpha.. The layer 81 is etched using the resist pattern 82 as a mask. After the resist pattern 82 is removed, the layer 81 having a taper is formed. In this method, the angle of the taper depends on the irradiation angle .alpha. of the ion beam 83. However, in this method, it is difficult to irradiate the ion beam 83 into the layer 81 for large irradiation angles. Therefore, it is difficult to make a taper having a sufficiently gradual angle to be applied to practical use.
FIGS. 8A and 8B show a method of forming a layer with a gradual taper by a wet etching method. A layer 81 is formed on a substrate 3, and a resist pattern 82 is formed on the layer 81 as is shown in FIG. 8A. The layer 81 is then etched using the resist pattern 82 as a mask by a wet etching method. A part of the layer 81 under the resist pattern 82 is also etched according to a, b, and c, as is shown in FIG. 8B. The layer 81 has a desired profile by stopping etching at a proper time. Using this method, the shape of the taper depends on an etching rate. However, the etching rate is subject to changes of agitation, concentration and temperature of the etching solution as will be appreciated. Therefore, this method does not provide good repeatability. Also, it is difficult to form a gradual taper using the wet etching method.
As is described above, a tapered layer is utilized for connecting two dielectric slab waveguides having different effective refractive indices by a conventional technique. However, it is difficult to form the tapered layer with a gradual angle and to produce the layer with good repeatability. The present invention overcomes aforementioned shortcomings associated with such conventional techniques and provides an optical device to connect two dielectric slab waveguides having different effective refractive indices without loss of light power.